As a parent of two small children (now coming up to 3 years old), my perspective is that pre-boarding is helpful both to the people concerned and probably to everyone else too. It's a major struggle getting small children, their car seats and all the sundry paraphenalia down the aisle, and when you get to your seats you can't necessarily just sit straight down out of the way. You have to attach the car seats to the lap belts, which can be awkward, while everyone is waiting in the aisle behind. And as you are likely to have an above average quantity of carryon, which really needs to be stowed closely and accessible, you need to be sure that space will be available. And this leaves aside the chaos that can ensue when the FA's belatedly declare on a full flight that your seats are unsuitable for young children (which has once happened to me in spite of efforts to confirm/resolve in advance.)

I do agree that the priority should only be given to very young children and one adult per child.